[³H]Adenosine transport in DDT₁ MF-2 smooth muscle cells: Inhibition by metabolites of propentofylline

Adenosine receptor signal transduction mechanisms have previously been characterized in Syrian hamster smooth muscle DDT₁ MF-2 cells but adenosine transport in these cells has not. DDT₁ MF-2 cells possess a high density (370,000 sites/cell) of high affinity (K(d) value of 0.26 nM) binding sites for [³H]nitrobenzylthioinosine, a marker for the equilibrative and inhibitor-sensitive subtype of nucleoside transporters. Transport of [³H]adenosine was insensitive to Na⁺ and was inhibited by the nucleoside transport inhibitors nitrobenzylthioinosine, dilazep and dipyridamole with IC₅₀ values of 1, 13 and 270 nM, respectively. Propentofylline, a neuroprotective compound that can inhibit nucleoside transporters, is rapidly metabolized in vivo to the racemate (±)-A720287. Based on recent findings that some transport inhibitors exhibit marked stereoselectivity, we tested the degree to which individual stereoisomers of (±)-A720287 affect adenosine transport. Propentofylline inhibited [³H]adenosine transport in DDT₁ MF-2 cells with an IC₅₀ value of 24 μM. (±)-A720287 and the individual stereoisomers (+)-833791 and (-)-844261 had similar potency to propentofylline for inhibition of [³H]adenosine transport in DDT₁ MF-2 cells as well as in clonal mouse leukemia L1210/B23.1 cells, cells which possess only the equilibrative and inhibitor-sensitive subtype of nucleoside transporters. Thus, the neuroprotective effects of propentofylline may be due, in part, to the primary metabolites of propentofylline.